LOS ANGELES — The atmospheric river storm soaking Southern California is expected to peak Saturday, with officials warning of potential flooding and damaging landslides, especially around recently burned areas.

Across a wide swath of the Southland, there’s a moderate risk for flash flooding, urban flooding and debris flow, a type of landslide in which rainfall pouring off hillsides can pick up mud and other debris with punishing speed, vaulting muck at speeds of up to 35 mph into homes and sweeping away parked cars.

There’s a small chance of particularly severe weather, including locally damaging winds and even a tornado.

Flood watches are in effect for some 20 million people — starting at 1 a.m. Saturday for Los Angeles, Ventura and Santa Barbara counties, and at 4 a.m. in all of Orange County and much of San Diego, Riverside and San Bernardino counties. The flood watches are expected to end throughout the region at 10 p.m. Saturday.

If the rain delivers as expected, Los Angeles could record its wettest November in 40 years, and register one-fifth of its annual average rainfall by the end of the weekend.

“We’re looking for a long duration of widespread rain, something on the order of 12 to 24 hours of really nonstop rain for much of Santa Barbara, Ventura and L.A. counties,” said Ryan Kittell, a meteorologist in the National Weather Service’s Oxnard office.

The agency’s Weather Prediction Center says large swaths of Southern California have a “moderate flash flood” risk on Saturday, defined as at least a 40% chance of intense rainfall that could lead to flash flooding. It’s unusual for the Weather Prediction Center to issue such an advisory for Southern California, Kittell said.

Given the forecast, officials urged residents to avoid nonessential travel, and warned those who must go out to never attempt to drive through a flooded roadway.

In L.A. County, the areas most at risk for rapid flows of mud and debris are the Eaton burn scar in Altadena, the Palisades burn scar in Pacific Palisades, and the Bridge burn scar north of Claremont. Officials are also closely watching the Line burn scar north of Highland; and the Airport burn scar, located between Rancho Santa Margarita and Lake Elsinore.

Evacuation warnings are in effect through 11 a.m. Sunday in areas near recent burn scars due to the risk of mud and debris flows. The warnings encompass areas near the Palisades, Eaton, Kenneth, Sunset and Hurst fires that burned in January, as well as the Airport fire that burned a year ago in Orange County.

Evacuation orders have also been issued to residents living in specified high-risk homes in Pacific Palisades and other recently burned areas.

As of 6 p.m. Friday, downtown Los Angeles had recorded 0.25 inch of rain, but could get an additional 2.6 inches of rain by the end of the weekend.

An additional 4 inches of rain could fall on the Bridge burn scar through Sunday, and about 5 inches is expected in the Eaton and Palisades scars. The bulk of the additional rain that arrives this weekend is expected to hit L.A. County over a 15-hour period, from 3 a.m. to 6 p.m. Saturday, Kittell said.

That’s an unusually wide window. In a traditional cold front that is pushed by the jet stream, forecasters can usually narrow the period of peak expected rainfall to between six and nine hours.

But this atmospheric river system has been cut off from the west-to-east jet stream, in what is known as a “cut-off low.” This type of storm, dreaded by meteorologists because it’s so unpredictable, spins like a top and could stall above Southern California for longer than a typical storm.

The rain will also come from a different direction than normal, bringing moisture up from the south rather than from the north, Kittell said.

Most people in Southern California will not experience dangerous flooding, Kittell said. But the risk does exist.

“The message is ... not to scare everyone, but certainly it’s best to be overprepared instead of underprepared,” Kittell said.

Forecasters expect a relatively narrow swath of land will be hit hard by more intense rainfall. Should that band of moisture hover over L.A. County, the Eaton and Palisades scars could be hit by rain falling at a rate of 1 inch per hour more — enough to cause significant debris flow and residential flooding.

There’s about a one-in-three chance of that happening in those recent burn areas, the weather service said.

But it’s also possible the swath of intense rainfall could hit Ventura or Santa Barbara counties, sparing L.A. County the worst. L.A. County is still heavily expected to get rain falling at a rate of half an inch per hour. At that rate, forecasters expect generally minor and shallow debris flows, as well as rockslides and mudslides in canyons.

Thunderstorms are another potential risk.

“Some of these could become severe, could become supercellular where they start spinning, and those carry a remote, non-zero, risk of damaging winds and/or a weak tornado,” Kittell said.

People who see lightning should stay inside and away from windows.

“There are a few steps you can take to stay safe during this storm. Consider changing your weekend plans and stay home, cuddle up, watch a movie,” Los Angeles Fire Chief Jaime Moore said. “Spend some family time.”

Should residents need to venture outside, Moore warned people to avoid walking or driving through moving water and to stay away from any downed power lines.

LAFD has pre-deployed resources across the city to respond to storm-related hazards. This includes a 22-member strike team with five fire engines patrolling the Pacific Palisades area, which is at high risk of mudslides and debris flows due to January’s fire, Moore said.

Urban search-and-rescue and swift-water rescue teams have been staged in the San Fernando Valley, while seven brush patrol vehicles are prepared to quickly reach narrow streets in high-risk terrain.

In San Diego, Orange, Riverside and San Bernardino counties, meteorologists say there’s about a 25% chance of thunderstorms as the storm moves through.

“If these thunderstorms do occur, we’re going to get locally higher rainfall rates, and that’s where we’re expecting the greatest threats for flash flooding,” said Dave Munyan, a forecaster with the weather service office in San Diego.

From late Friday through Sunday, 1.5 to 2 inches of rain could fall on the cities of San Bernardino and Riverside. Irvine, San Diego, Oceanside, San Clemente and Escondido could see 2 to 2.5 inches; and Anaheim could get 2.5 to 3 inches. In the deserts, Palm Springs could get 0.7 to 1 inch of rain, and Joshua Tree National Park could see 1 to 1.5 inches.

During that same time period, Pomona could get 2.29 inches of rain; Long Beach, 2.53; Oxnard, 2.6; Santa Barbara, 2.82; and Ojai, 3.98.

Delays are expected at Los Angeles International Airport on Saturday. A few inches of snow may fall at the resort levels of Southern California’s mountains.

“Not a whole lot of snowfall is expected,” Munyan said.

A weather model forecasts this atmospheric river as an AR Category 3, a strong atmospheric river that is a balance of being beneficial and hazardous, according to the Center for Western Weather and Water Extremes at UC San Diego’s Scripps Institution of Oceanography. There are five categories: 1 being the weakest and primarily beneficial, and 5 being exceptional and primarily hazardous.

Two more storms are potentially on the horizon — one Sunday night through Tuesday, and another Wednesday night through Friday. The most likely outcome for both of those storms is low amounts of rain, with less than 1 inch of rain expected along the coast and foothills, forecasters say.

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